This invention relates to sheet material, especially a base sheet obscured by an opaque but transparentizable microporous, diffusely light-reflective layer.
For centuries paper has been one of the most versatile substances made by man. Formed from commonly available cellulosic materials, it can be made stiff or flexible, rough or smooth, thick or thin, and provided with any desired color. After it has served its intended purpose, it can often be repulped and used again. In recent years, however, the demands for paper have increased to the extent that it has finally been recognized that the sources of cellulosic raw materials are not inexhaustible. Further, the energy required to manufacture paper is a significant consideration in a world becoming increasingly aware that supplies of energy are also finite. It has also become recognized that, where paper is used as a carrier for inidicia, it can generally be used only once, it being impossible or impractical to remove indicia which are no longer needed or desired. There has thus arisen a desire for a substitute for paper, especially one which can be repeatedly and easily reused; even a substitute which was more expensive to manufacture would be less expensive in the long run if it could be reused a sufficient number of times.
Several U.S. patents (e.g., Kallock U.S. Pat. No. 2,299,991, Larsen U.S. Pat. No. 3,031,328 and Thomas U.S. Pat. No. 3,508,344) disclose composite sheet material wherein a light-colored opaque blushed lacquer layer is coated over a base sheet which is either dark-colored or imprinted with dark-colored indicia. The opacity and light color of the blushed lacquer coating are due to the inclusion of numerous microvoids; the local application of (1) heat or pressure (either of which irreversibly collapses the microvoids) or (2) a non-solvent liquid having substantially the same refractive index as the lacquer (which fills the microvoids), causes the coating to become selectively transparent and the underlying dark backing to become visible. A non-solvent liquid employed to impart transparency to the opaque microporous layer can subsequently be evaporated to restore the original appearance. A liquid which was a solvent for the lacquer coating would, of course, result in permanent transparency by collapsing the microvoids.
Phillpotts U.S. Pat. No. 2,854,350 describes structures which are functionally similar to those just described, except that the blushed lacquer coatings are replaced by a microporous layer of finely divided calcium carbonate in an organic binder. Transparency is imparted by locally applying pressure or treating selected areas with a wax, oil or grease having a refractive index similar to that of the calcium carbonate. Other pigments may be incorporated in a microporous highly plasticized resin binder; see Hoge et al. U.S. Pat. No. 3,247,006.
It is sometimes desirable to have microvoid-containing sheet material which can be transparentized by applying a liquid, but which cannot readily be transparentized by the application of heat or pressure. In such circumstances, a microvoid-containing layer of the type described in Arens U.S. Pat. No. 4,299,880, owned by applicant's assignee, is preferred. This patent discloses a structure in which the microvoid-containing layer consists essentially of particles held in pseudo-sintered juxtaposition by a thermoset binder and has a cohesion value of at least 400 grams force*. FNT *The cohesion value is determined by knife-coating a dispersion of a putative composition on a cleaned gray cold rolled steel panel, drying and curing as appropriate for the composition, to provide a coating 50-60 micrometers thick. Using a "Balance Beam Scrape-Adhesion and Mar Tester", sold by Gardner Laboratories, Inc., Bethesda, Maryland, a sapphire-tipped stylus is lowered into contact with the test panel and held in fixed position while a ball bearing-supported platform moves the panel. The minimum grams-force required to form a 50-micrometer deep scratch in the coating in a single pass is determined at a magnification of 40.times. and reported as cohesive value.
To a greater or lesser extent, each of the products described in the preceding paragraphs suffers from the disadvantage that localized application of a transparentizing liquid results in an image which does not maintain its original sharp outlines with the passage of time. In other words, there is a tendency for the marking liquid not only to penetrate the microvoids perpendicularly to the surface but also to wick laterally. As a result, the longer the transparentizing liquid remains in contact with the microvoid-containing layer, the less distinct the original image. In some instances, it becomes difficult to distinguish similar numerals (e.g., 6, 8 and 9) or letters (e.g., l, i and t) from each other. Prior to the present invention, no way of combatting this problem was known.